Thermo-elastic behavior and P/T phase stability of TlAlSiO4 (ABW)

The P/T phase stability and the thermo-elastic behavior of synthetic TlAlSiO4 [ABW framework type, space group Pna21, a = 8.2719(2), b = 9.4373(2), c = 5.4180(1) Å] have been investigated up to 950 °C (at room-P) and up to 8 GPa (at room-T) by means of in situ synchrotron powder diffraction with a diamond anvil cell and with a high-temperature furnace. The Rietveld structure refinement of TlAlSiO4 at room P/T confirms the general structure model previously reported. Only one independent Tl site, with full site occupancy and close to channel wall, represents the extra-framework population. No phase transition has been observed within the temperature- and pressure-range investigated. P–V data were fitted to a second-order Birch–Murnaghan Equation of State (BM-EoS), giving: V0 = 420.76(5) Å3, KT0 = 48.8(2) GPa. The evolution of the lattice parameters with pressure shows a significantly anisotropic compressional pattern. The elastic parameters calculated with a “linearized” second-order BM-EoS are: KT0(a) = 21.96(7) GPa for the a-axis; KT0(b) = 68(1) GPa for the b-axis, and KT0(c) = 112(2) GPa for the c-axis. The volume thermal expansion with T was modeled by the polynomial function: V(T)/V0 = 1 + α0·ΔT + α1·ΔT2 = 1 + 4.44(3)·10−5·ΔT − 2.3(3)·10−9·ΔT2. The anisotropic thermal scheme is characterized by a negative thermal expansion along [010] (i.e. α0(b) = −8.5(1)·10−6 °C−1), almost no expansion along [0 0 1] (i.e. α0(c) = 0.9(1)·10−6 °C−1) and a positive expansion along [1 0 0] (i.e. α0(a) = 52.4(1)·10−6 °C−1). A comparative analysis of the thermo-elastic behavior of the isotypic TlAlSiO4 and CsAlSiO4 is carried out.